The present invention relates to an improved lighting unit. More particularly, the present invention relates to an improved lighting unit of the type incorporating a gas discharge tube and having improved electrodes effective for facilitating the starting of the gas discharge tube and an improved ballast circuit cooperating with the improved electrodes.
Recent improvements to the incandescent art have provided an improved lighting unit having a gas discharge tube as a main light source and also an incandescent filament as a supplementary light source. Such an improved lighting unit is generally disclosed in U.S. Pat. No. 4,350,930, of Peil et al, issued Sept. 21, 1982 and assigned to the same assignee as the present invention.
The gas discharge tube disclosed in U.S. Pat. No. 4,350,930 has various modes of operation such as (1) an initial high voltage breakdown mode, (2) a glow-to-arc transition mode, and (3) a steady state or run mode.
The run mode of operation of the gas discharge tube is considered the best mode for improving the operation of the lighting unit. During this run mode the electrodes within the gas discharge tube operate in a thermionic hotspot mode in which the electrodes are heated to thermionic temperatures so as to provide a thermionic arc and thusly supply the lighting unit with a high efficiency gas discharge tube serving as its main light source. It is desired that the improved lighting unit achieve this run mode as quickly as possible. This achievement is hindered in that the gas discharge tube is necessarily first sequenced through the glow mode in order to establish the run mode.
A ballast circuit of the improved lighting unit provides the necessary conditions to place the gas discharge tube into the glow mode. These conditions are a voltage having a typical value of 150 volts and a current having a typical value of 20 ma, both of which conditions serve to provide power to heat the electrodes of the gas discharge tube so that the electrodes attain and sustain the desired thermionic arc conditions required for transition into the run mode.
The power supplied to the gas discharge tube is supplied by an electronic ballast circuit which may be such as disclosed in the aforementioned U.S. Pat. No. 4,350,930. The voltage and current conditions applied to the gas discharge tube for the breakdown, glow, and run modes of operation are developed by a ballast circuit of the lighting unit. It is desired that the necessary voltage and current conditions for the gas discharge tube be reduced so that the implementation of the ballast circuit itself may be correspondingly reduced in size and simplified. One possible way of reducing size is to eliminate the glow mode operation of the gas discharge tube, which, in turn, allows the reduction of the number of circuit components needed.
Elimination of the glow mode is accomplished in the present invention by substantially improving the characteristics of the electrodes to achieve and sustain thermionic emission for the gas discharge and also by adapting the ballast circuit to the necessary operation of the improved electrodes. It is desired that electrodes be provided having improved characteristics so as to sustain a thermionic arc in the run mode without the need of first being preconditioned by glow mode operation. If the glow mode operation is eliminated, a two-fold benefit is achieved in that (1) the implementation of the ballast circuit can be simplified, and (2) the improved lighting unit more quickly achieves its most beneficial mode of operation; namely, the run mode.
Accordingly, objects of the present invention are to provide (1) a gas discharge tube having electrodes with improved characteristics effective to sustain a thermionic arc in the run mode without first being preconditioned by the glow mode of operation, (2) a simplified ballast circuit adapted to the improved characteristics of the electrodes of the gas discharge tube, and (3) an improved lighting unit which quickly obtains its run mode of operation.